Electrically and acoustically excited hearing aid



Nov. 21, 1944. F. M GROSSMAN ELECTRICALLY AND ACOUSTICALLY EXCITEDHEARING AID Filed Aug. 26, 1942 IN VEN TOR. mama/(K M. G/ OISMAA/ @fnyJ& My

Patented Nov. 21, 1944 ELECTRICALLY AND ACOUSTICALLY EXCITED HEARING AIDFrederick M. Grossman, New York, N. Y. Application August 26, 1942,Serial No. 456,162 Claims. {'01. 179-107) This invention relates toelectrically and acoustically excited hearing aids for individuals withimpaired hearing function and it is working on the principle of airconduction, i. e. sound transmission through the air in the external earcanal.

Electrically excited hearing aids of this type are well known in theart. They comprise principally a microphone for picking up andtransforming sound waves into corresponding electrical energy usuallyderived from a battery, an amplifier for this energy and a receiver fortransforming again the electrical energy into acoustical energy. It isconvenient to mount the receiver on a mold of plastic materialindividually shaped to lit the outer ear and projecting into theexternal ear canal; the mold is provided with a channel which at one endopens into the ear canal, while at its other end it opens toward thereceiver.

As outlined in the paper entitled Some Problems Involved in the Fittingof Hearing Aids by Halstead and the inventor (N. Y. State Journal ofMedicine, vol. 41, issue No. 4 of Feb. 15, 1941), hearing impairmentscan be classified in four classes which requires specific and difierentcharacteristics of the hearing aid.

This invention resides substantially on the following considerations andexperience.

The resonance of the external ear canal is important for the perceptionof the high frequency partials contained in the speech sound spectlum.It is therefore necessary that the acoustical characteristic of theexternal ear canal be changed as little as possible by a hearing aid.

Low frequencies contained in the sound spectrum are chiefly responsiblefor the loudness sensation of speech sound. Approximately 95% of speechpower is attached to frequencies below 1000 cycles in the spectrum ofspeech sound (cf. Dunn and White, Journal of the Acoustic Society ofAmerica, vol. XI, 1940, p. 278). Persons suffering from the variabletype hearing loss, (cf. Steinberg and Gardner, Journal of the AcousticSociety of America, vol. DI, p. 11, 1937) have of necessity a narrowerband of comfortable loudness sensation as compared to the intensity bandoflered.

It is essential to equalize peaks of speech power with parts of lowerspeech power. This can be done efiectively by attenuating the lowfrequency content of speech sound.

It has further been observed that loud speech sounds which must containlow frequency partials of relatively higher intensity so as to be loud,produce with individuals with perceptive hearing loss the sensation ofbanging." Low frequency partials also mask the high ones and are chieflyresponsible for the loudness sensation of speech sound. High frequencypartials are, however, indispensable for the intelligibility of speech.The perception of high frequency partials is impaired in perceptivehearing loss so that the masking eflect is multiplied. Since theperception of high partials is reduced in individuals sufiering fromperceptive hearing loss and the loudness sensation in those individualsis pathological inasmuch as the intensity band producing comfortableloudness sensation is narrower, the low frequency content of speechsounds transmitted to those individuals should be reduced.

It has therefore been suggested to translate to individuals sull'eringfrom perceptive hearing loss and impairments of similar eiiect, thespeech sound through an electrical filter in such a manner that the lowfrequency partials of the sound are amplified to a lesser degree thanthe high frequency partials. Surprisingly it appeared that sucharrangements did not have the expected result. The banging could not beprevented or reduced to a tolerable extent.

The inventor believes to have discovered the reason for the failure ofthis known arrangement. While it is possible to change the intensitiesof different frequencies in a speech sound by means of an elaborateelectrical filter and thus to deliver to the receiver a changed soundspectrum which theoretically suits the conditions of the individualsufl'ering from perceptive hearing loss, the sound emanating from thereceiver and passing the channel in the mold before discharging into thepart of the ear canal left free by the mold, is again being changed inits qualities.

As it has been shown by Stewart (Acoustics, by Stewart and Lindsay,1930, p. 159) any constriction in a channel through which sound wavesproceed, results in attenuation of higher frequencies, while lowerfrequencies pass such channel with unchanged intensity. Consequently thesound emanating from the receiver and which contains low frequenciesattenuated in intensity by the electrical filter, are now subject toattenuation of the higher frequencies while proceeding through theconduit composed of the narrow channel in the mold, which forms aconstriction, and the far wider free part of the ear canal. Thus the perse favorable filtration of the speech sound by the electrical filter issubstantially neutralised by the filtration to the opposite effect inthe acoustical low pass filter formed by the constricted channel in themold followed by the far larger part of the external ear canal.

It is therefore an object of the invention to provide the acoustic linefrom an electrically controlled sound generator of an air conductionhearing aid to the ear, acoustically operating means for filtering thesound translated along that line in such a manner that it becomesintelligible and comfortable for individuals suffering from perceptivehearing loss and similar ailments.

It is still another object of the invention to provide an air conductionhearing aid with an adjustable acoustical filter which rendersintelligible the sound proceeding from the sound generator of thehearing aid to the ear of an individual suffering from perceptivehearing loss and similar disabilities, and permits adjustment of thefrequency response characteristic of the sound even without thecompulsory use of electrical filters.

It is still another object of the invention to provide the acoustic linefor an electrically controlled sound generator of an air conductionhearing aid to the ear with acoustically operating means for filteringthe sound translated along said line and to translate this sound to thecar without considerable change in the acoustic characteristics of theexternal ear canal.

These and other objects of the invention will be more clearly understoodwhen the specification proceeds with reference to the drawing in which afew embodiments of the invention are shown by way of exemplification.Conventional elements of electrically controlled air conduction hearingaids are omitted in the drawing for sake of simplification, for instancethe microphone, the source of electrical energy, the amplifier, meansfor regulating the amplifier (or set of amplifiers) and heating theircathodes and feed ing their plate circuit, and conductors between them.Furthermore, while an electromagnetically controlled membrane ordiaphragm is shown as sound generator in the receiver, it should beunderstood that any other sound generator, such as of the crystal typecan be used to the same effect.

Referring to the drawing, Fig. 1 shows in perspective view and withparts in cross section a hearing aid according to the invention fittedto the ear; Fig. 2 on a larger scale a cross section with parts inelevation through the hearing aid: Figs. 3 and 4 cross sections alongline 3-3 in Fig. 2 in d fferent relative positions of two elements ofthe hearing aid; Fig. 5 a perspective view of an element of the hearingaid attached to a part of the receiver; Fig. 6 a perspective view ofanother element of the hearing aid attached to an ear piece; and Fig. 7a perspective view of a modification of the element shown in Fig. 5.

Referring to Fig. 1, a receiver ID of an ear con duction hearing aid ofconventional structure is shown, provided with conductors H leading tothe other conventional elements of such a hearing aid. An acoustical andpreferably adjustable high pass filter I 2 is associated andsoundtightlyconnected at one end with receiver I0, and at the other end with an earpiece or mold i3 which is fitted into the external ear canal It. Anyother suitable fitting of the free end of the filter l2 to the ear canbe used.

Referring now to Figs. 2 to 6. it will be appreciated that the receiverin comprises a membrane 76 or diaphragm l5 held or clamped betweenadiacent surfaces of the casing or any other element of the receiver,and its cover l6.

An element of an acoustical high pass filter is conveniently connectedwith cover it or made integral with it, as shown. It consists of atubular member l1 provided with a number, one as a minimum, of holes IQof equal or difierent diameters. The tubular member I! slightly taperson its inside towards cover l6. Another tubular member I! snugly fitswith its tapered outside into tubular member I! and is cut out at 28. inthis exemplification over about half its circumference. It is connectedwith the ear piece 13 in any suitable way or made integral therewith asshown. While the telescoping tubular members H and I9 may be made ofmetal and connected. respectively with cover l6 and earpiece l3 in anysuitable way, member I9 if it be made integral with ear piece l3consists of the same material as the ear piece, e. g. of moldableplastic material.

A circumferential groove 20 is provided near the free end of tubularmember l9 into which the inner edge of an annular spring element 2|snaps when member I9 is assembled with member 11. The conical portion ofmember 2| is slotted radially so as to permit that snap action. Therebythe tubular members I! and iii are kept permanently in their relativeposition, as shown, permitting relative rotation to one another.

By turning tube I! relative to tube l9 into a position as shown in Fig.3, all the holes or orifices l8 are communicating both with the interiorof the tubular members as well as with the surrounding air.

By turning those members into the position shown in Fig. 4, for instanceone of the orifices i8 is covered by the solid portion of tubular memberi9.

It will be observed that in other relative positions of the two tubularmembers any desired number of orifices i8 can be covered or left openand thereby become effective.

With increasing number of side branch orifices in series, the degree ofattenuation and range of attenuated low frequencies is increased; with asmaller number of orifices the attenuation decreases gradually withincreasing frequency.

By arrangement of more than one orifice in parallel, i. e. in one planesubstantially vertical to the axis of the conduit, the attenuatingeffect can be increased while the band width of low frequencies soaffected remains substantially the same.

It is a further condition of the filtering action of a conduit providedwith side branch orifices that their diameter or dimension of a slotmeasured in the direction of the axis of the conduit. is considerablysmaller than a wave length to be attenuated, and if orifices arearranged in series. that their distance measured in the direction of theaxis of the conduit be small compared with the wave length to-beattenuated.

Applying these per se known acoustical laws to the conduit formed by thetubular members l1, l9 and the side branch orifices l8 provided thereinin a plane substantially vertical to the axis of the conduit, it isclear that the diameter of holes IE, or if there be provided elongatedslots instead, the largest dimension of such slots in the direction ofthe axis of the conduit should be reasmaller than a wave length to beattenuated. Ex-

pe'riments show that holes ll of a diameter of about A to of amillimeter gave satisfactory results, though larger diameters can beused under observation of the laws of Stewart.

Also the wall thickness of member I! should be such that the orifices I!are reasonably short and the desired effect is invariably obtained, andtherefore advantageously between about V to 2 /2 mm. Too thick a wallmight becloud the proper operation of the side branch orifices.

The total axial length of the conduit formed by the telescoping tubularmembers II, It should also be made as short as possible in order toeliminate the effect of phase differences, and for the further reasonthat thereby the hearing aid is rendered less conspicuous.

The ear piece II is made as short as possible in axial direction andprojects as little as possible into the external ear canal l4,compatible with firm seating of the ear piece in the ear. Care shouldalso be taken that the inner diameter of the mouth 29, Figs. 2 and 6, ofthe ear piece is as large as possible and approximates that of theexternal ear canal, so as to avoid constrictions in the acoustical linefrom the sound generator, e. g. membrane ii to theear drum. such constrictions, if large enough, could detrimentally affeet the fidelity ofthe sound translated along it, since constrictions in an acoustical lineor conduit render it a low pass filter. By making the cross sectionalarea of the conduit formed by members l1, l9 and of the mouth piece ormold I; approximately the same as the cross sectional area of theexternal ear canal, detrimental distortions or low pass filter effectsare avoided. By making the ear piece as short as'possible, it occupiesan acoustically almost negligible part only of the external ear canalwhereby the resonance characteristics of the latter which are essentialfor true translation of the sound are essentially retained.

By adjusting, i. a. turning the tubular members i1, is relative to oneanother. any desired number of side branch orifices 18 or portion of oneorifice can be made operative or eliminated, and thereby any desireddegree of attenuation of low frequencies and consequently any desireddegree of comfort and intelligibility of the translated sound offered tothe individual fitted with this hearing air can be realized. Suchadjustment will be made preferably by the expert who fits the hearingaid to the ail ng individual; the latter may also bring about slightadjustments to accommodato himself to particular conditions of loudnessof sound to be perceived, which also depend on and change with thearchitectural acoustics of the room in which the individual stays.

In the event that in addition to degree of attenuation also the width ofa low frequency band is intended to be influenced. two (or more) rows oforifices I8, 28, Fig. 7, may be provided, and member i9 and its out outwindow 28 shaped so that its solid portion covers or its window uncoversthe same or different numbers of orifices in each row in the diflerentadjusted relative positions of member is to member II.

It will be understood that window It may be provided in member I! andthe orifice or orifices II (and 26) in member l9.

It will also be understood that instead of a. membrane 15, electricallycontrolled by electromagnet 21, any other electrically controlled soundgenerator can be provided, such as of the crystal type.

The outstanding advantages of the invention consist in its simplicity,negligible weight and inexpensiveness, doing away with complicated andcomparatively heavy and expensive electric filters. If electricalfilters are used for any particular reason, they can be simple and ofreduced effectiveness. The acoustical hish pass filter used by theinvention can more easily be adjusted than an electrical filter,particularly by the user. Any suitable degree of attenuation and bandwidth of attenuated low frequencies can be obtained. With any number oforifices arranged in one or two rows always smooth curves representativeof the degree of attenuation dependent on frequency, Fig. 8, arerealized and undesirable cut-oils are eflectively avoided, whileelectrical filters often produce such cut-oil's. The acoustical line orconduit formed by the acoustical filter and channel of the ear piece canbe made short and straight, its cross sectional areas can be madeuniform over its entire length and almost the same as of the externalear canal, whereby most efilcient sound translation is secured anddistortion of the resonance characteristic of the external ear canal isavoided.

It should be understood that my invention is not limited to theexemplification referred to herein, but to be derived in its broadestaspect from the appended claims. The reference in some of the latter to"side apertures or "side orifices should be understood in the light ofthe foregoing specification to define apertures or orifices, includingslots, the dimensions of which (particularly measured in the directionof the axis of the sound conducting conduit or channel with which theycommunicate) are reasonably small and in any event considerably smallerthan the length of a sound wave to be attenuated, in order to establishan acoustical high pass filter.

What I claim is:

1. An air conduction hearing aid including an electrically controlledsoimd generator and hollow means associated therewith so .that soundgenerated by said generator is propagated through the interior of saidhollow means the free end of which is adapted to be fitted into anexternal ear canal. characterized by the novel feature that said hollowmeans are provided between said generator and end with a number, one asa minimum, of side apertures establishing open communication between theinterior of said hollow means and the surrounding atmosphere andconstituting with said interior an acoustical high pass filter, and thatadditional means are provided for adjusting the effective area of saidapertures.

2. An air conduction hearing aid including an electrically controlledsound generator and hollow means associated therewith so that soundgenerated by said generator is propagated through the interior of saidhollow means the free end of which is adapted to be fitted into anexternal ear canal, characterized by the novel feature that said hollowmeans are provided with a number, one as a minimum, of side aperturesestablishing over the shortest way and essentially perpendicular to theaxis of said hollow means, open communication between the interior ofsaid hollow means and the surrounding atmosphere and constituting withsaid interior an acoustical high pass filter; that the maximum extensionof any such aperture measured co-axially with said interior does notexceed about two and one-half millimeters; and that additional means areoperatively connected with said hollow means for ad justing theeifective area of said apertures.

3. An air conduction hearing aid including an electrically controlledsound generator and hollow means associated therewith so that soundgenerated by said generator is propagated through the interior of saidhollow means the free end of which is adapted to be fitted into anexternal ear canal, characterized by the novel feature that said hollowmeans are provided between said generator and end with a number, one asa minimum, of Side apertures establishing open communication between theinterior of said hollow means and the surrounding atmosphere andconstituting with said interior an acoustical high pass filter; that thecross sectional area of the interior of said hollow means from saidgenerator to said free end is essentially constant and approximates thecross sectional area of an external ear canal to which the hearing aidis to be fitted; and that additional means are provided for adjustingthe effective over-all area of said side apertures.

4. An air conduction hearing aid, substantially comprising, incombination, an electrically controlled sound generator exemplified byan electromagnetically controlled membrane, two telescoped tubularmembers adjustable one relative to the other, at least one of saidmembers associated with said generator so that sound produced by thelatter is translated through the inner one of said members, one of saidmembers provided with a number, one as a minimum, of side apertures themaximum extension of any such aperture measured co-axiall'y with saidmember not exceeding about two and one-half millimeters, the other oneof said members provided with one or more passages arranged so that theycan be aligned with one or more of said apertures in the other memberupon relative adiustment of said members, and an ear piece associatedwith the free end of at least one of said members, said ear pieceprovided with a channel communicating with the interior of the izmer oneof said members.

5. A hearing aid as described in claim 4, in which the cross sectionalareas of the interior of the inner one of said tubes and of said channelapproximate the cross sectional area of an external ear canal to whichthe hearing aid is to be fitted.

FREDERICK M. GROSSMAN.

